Race Car Aerodynamics - Short Track Aero Drag

1/10A lot of aero development work goes into the design of a modern prototype race car. We don't have the advantage of sophisticated computer simulation programs, but we can learn from those who do.

I'm sure that everyone has at one time or another been caught doing this. It is also possible that your parents told you to stop doing it or bad things were going to happen. Clearly, I am talking about sticking your hand out the window of a moving car. Parents can be such buzzkills! Until you were told to stop, you could feel the force of the air against your hand. It was something unseen, but you knew there was a force there. If you placed the palm of your hand at a right angle to the ground, you could feel the force against your hand and arm.

Conversely, by placing your hand parallel to the ground with your thumb or little finger cutting into the wind, you would feel an instant reduction in force against your arm and hand. You could change the angle of your hand and drive your arm up or down, depending on the position of your hand. You also knew that the faster your dad or mom would drive, the greater the force on your arm would be. This simple experimentation was your first foray into aerodynamics. You were experiencing the force of air-even if you didn't know it at the time. From an engineering perspective, you were feeling the force of a fluid as it acts on a solid object moving through it.

2/10The drag that comes with running large wings may well offset the downforce advantage. A better designed wing might not need such a high angle of attack to the wind.

There is a great deal of energy in moving air. Just look at the damage that can be caused by a storm or the unfortunate consequences of hurricanes that have recently plagued the Southeastern portion of the United States. Just as there are great amounts of energy in moving air, a great deal of energy is expended when moving your race car through the air by the simple act of accelerating the car around the track. If you can reduce the amount of parasitic drag draining your energy bank, you could use that surplus energy (horsepower) to accelerate the car.

We all have listened to the race announcers talk about aero-push, slipstreaming or drafting, and getting the car in dirty or clean air. It is clear that on larger tracks there is a very strong effect on the car as it moves through the air. If we add more cars to the picture, the situation gets more complex and exciting. What many racers do not realize is that the effects of aerodynamics start to act on the car at speeds as low as 45 mph. Many short-track racers do not even think about the forces that the air generates for and against their cars. They seem to focus on the more visceral aspects of short-track racing. We all know that cars can reach speeds of 90 to 100 mph on a quarter-mile track, and the speeds are even higher on half-mile tracks. The cars will go even faster with added banking.

3/10As the air flows over the car, the shape and smoothness of the body determines how much drag it creates. For a lower Cd, we need to have a polished surface that is free of protruding objects such as bolt heads, hood edges, crash wrinkles, and so on.

Let's spend some time talking about air and the speed you can gain by learning to work with the flow instead of going against it. First, as with any discussion, we need to have a common language. Please remember that air acts like a fluid, but it is compressible and its density varies as the weather changes.

Aerodynamics: The study of the motion of fluids (in this case air) on an object or objects and the forces created by the motion of the fluid or the motion of structures within the fluid. This is a very high-level definition. For the purposes of this discourse, we are concerned with the forces generated on a race car as it passes through the air.

Airflow/Air stream: The flow of air around the race car-front, sides, top, and bottom.

Cd: The coefficient of drag or drag coefficient. This is expressed as a number that represents the amount of drag a body creates as it passes through the air. The lower the Cd, the better. It is commonly expressed in numbers less than 1.

4/10Note the small frontal area presented with this Daytona prototype running in the Grand Am Series. The air "sees" very little and the flow over the slick body is smooth and uninterupted.

Drag: A force that acts against a body moving through a fluid. In our case, the fluid is air. Drag is the friction or resistance that the air generates as the body moves through it. Drag acts in an opposite direction to airflow. In our application, it's opposite to the direction the car is heading. The lower the Cd, the lower the drag that is generated as the car is moving through the air. Drag has two components: parasitic drag and induced drag. These constituents of drag are all contributory to total drag. This is a bad thing. We want to try to reduce drag if at all possible.

Downforce: A downward force generated as a body moves through a fluid. As racers, we need to balance the amount of drag versus the amount of downforce. The two are very different, and the results of each are very different in respect to improving vehicle performance. In an ideal world, we would try to increase the downforce and reduce the drag. An increase in downforce does not necessarily indicate that we will be able to reduce drag. Conversely, an increase in drag does not mean a decrease in downforce. The two are linked but not mutually exclusive.

5/10While we tend to think that round is a smooth shape that would offer a good aero surface, it is clear that it is not. The airflow is very turbulent as it flows past the tube. This has a greater coefficient of drag. The turbulence is even greater as air flows past an open cage, as on a Sprint Car.

We have all seen the huge wings on the tops of Sprint Cars and the large side boards that the Late Model racers utilize. These aerodynamic devices are used to help keep the cars stuck to the track as they rotate through the corners. The Sprint Car has a wing that has the same general shape used on an airplane, except it is mounted upside down. So, instead of creating lift, it is creating downforce. In all actuality, it is still creating lift, but the force is in the opposite direction. The principle is the same, but we pay a price for generating that downforce-we get a great deal of drag. Consequently, it takes some very expensive engines to push the cars down the track. My dad always told me there is no such thing as a free lunch.

Remember, in race car design and application, we are not trying to create lift. We are creating downforce with very little drag. That is the key-low drag and high downforce. This should be your new mantra.

Laminar Flow: The air is moving in smooth layers around our car. In this case, the air can be likened to a gently curving country road. The curves are smooth and gentle, so it is not necessary to slow down as you progress through them. That is very much like laminar flow. The illustration is somewhat simplistic, but you get the general idea. Air takes less energy to move over smooth surfaces. That is why the bodies on race cars that race at high speeds are so sensitive to small changes in shape, such as the changes caused by contact (e.g., rubbing and bumping).

6/10With the addition of a small fairing to the tube, the airflow across the tube is much smoother. Consequently, the drag numbers go down and it takes less power to push this faired tube through the air.

Turbulent Flow: The flow is random in its shape. The air forms currents and erratic flow patterns that are contrary to the majority of the flow. The flow is disordered. Many times, this is the type of flow that our head porter brethren are trying to achieve. In actuality, they are trying to reach a balance between laminar and turbulent flow through the ports.

Boundary Layer: The molecules of air are not moving in this layer, which is usually adjacent to the surface of the body moving through the air. The depth of the boundary layer is dependent on several factors: the density or viscosity of the gas, the finish of the surface moving through the fluid, and the compressibility of the fluid (which in our case is air). For our uses, the thinner the boundary layer the better. A larger boundary layer impedes the progress of the air more than a thinner boundary layer. This is the reason that it is so important to keep the surface of the car as smooth as possible. On dirt, this is a great reason to keep the car as clean as possible between heats. A clean car not only makes your sponsor's logo more visible to the fans, it also makes a difference in performance.

Frontal Area: This is the first portion of the car the air contacts. It's a cross section of the frontal portion of the car. Visualize your race car backed up to a garage door. There are floodlights shining on the front end, and the shadow that the car casts against the garage door is the frontal area.

7/10Notice how aero dirty the front of this Sprint Car is. The shock, axle, roll bars, brakes, and so on are hanging out in the wind. Rules may prevent some changes, but whatever we can do to clean up our cars in an aero sense would be worth it.

Anything within the structure and limitations of the rules that you can do to reduce the frontal area will improve the performance of the vehicle. If you can massage the roof line on your Modified or Stocker within the framework of the rules, it is a positive. If your racing division uses templates, make sure that your car fits the templates but is as close a fit as possible.

Racers tend to be very vocal when it suits them. There are many complaints about the cost of racing, and there is always someone or something to blame for the cost of speed. It would seem that if they could buy a few tenths for little or no money, it would generate some buzz. If you arrive early at any short track across the country and walk through the pits, you can see many racers giving away speed without even knowing it.

Ill-fitting body panels, large bolt heads in the air stream, exposed tubes, spoilers that are too large or have nothing to do with making the car go faster, suspension parts just hanging into the air stream, and tires sticking out past the edges of the fenders are all just waiting to rob you of the speed you work endless hours to achieve. Granted, many of these appendages are a required part of the car and serve a purpose, but some further thought could go into how these parts are integrated. It would be nice if, in addition to serving a valid function, they did not add to the overall drag coefficient of the car.

8/10This adjuster cylinder helps the driver adjust to track conditions so that when the track becomes more sticky, the wing can be lowered to a lesser angle of attack to the wind and therefore create less drag.

In the first example of our hand in the wind, we were able to adjust the amount of force that we could feel by changing the position of our hand in the air stream. We adjusted the drag and lift with one simple movement. We can also affect our race car's ability to move through the air and reduce the amount of power required to move the car down the track by adjusting the way the air interacts with the car. If we reduce the drag, the power that was previously used to overcome the drag can be used to accelerate the car.

One of the easiest ways to reduce the power-robbing effects the air has on our racing vehicles is to reduce the frontal area. This can be accomplished in a number of ways. The simplest way would be to make the car smaller. This is not always possible due to rules and the use of templates by the technical community to ensure the cars are all the same. If you can, make the car lower while still meeting the letter of the rules-you need to do it! Lower the roof, and lower the car to the track within the rules and the boundaries required to keep the car from hitting the track.

If the rules say the car has to be 4 inches off the ground, that is where you put it-not 5 or 4 1/2, but 4. While this may seem to be a simplistic solution, this activity can significantly reduce the frontal area of your car. It will reduce the size of the hole in the air that the car will be required to penetrate as it moves down the track. The larger the hole, the greater the horsepower required to move the car. As a direct result, you will have less horsepower in surplus to accelerate the car.

9/10This prototype has a splitter piece that the air flowing under the car must flow past. It is specially shaped to direct the airflow. Small 1/8-inch changes in the height of this piece affects the handling of the car.

You also want to keep the air from under the car. For the most part, you will not find ground effects cars at the local short track. The point is to keep the air on the outside of the car. This includes the air that will go through the radiator. Get it in and then get it out. If your rules allow it, you should try to duct the air back to the outside of the car, not under it. I realize this is difficult to accomplish, but there is an advantage gained by doing this. Not long ago, I saw some Sprint Cars that had special radiators off to one side. The ducting away from the radiators dumped the air out along the side of the hood, and the air intake was on the opposite side of the car. Air in, air out-quite a clever bit of engineering!

Every surface that interacts with air should be as smooth as possible. All body joints need to be smooth, with no edges or overlapping seams facing the airflow. Look at your car and see what areas you might improve from an aero perspective. While many would like to think of this as rocket science, at these lower speeds it is more common sense than science. As the speeds increase, the need for greater levels of science is required.

At short-track speeds (below 100 mph), it is more imperative to make sure you are not giving anything away due to overlooking an area or poor preparation. You need to look at large spoilers, air dams, and air fences as devices that may not be necessary at low speeds. Rear spoilers greatly deprive a race car of power. If you don't need one, you may consider removing that 4- or 5-inch spoiler. At the very least, do some on-track testing to determine if the spoiler makes a difference. The amount of downforce a spoiler makes may be insignificant when balanced against the potential drag.

10/10The control arms on this prototype are shaped to be very aero efficient. While this shape is not needed on this full-bodied car, these arms are also used on open-wheeled Formula cars.

From your perspective, we need to make sure we are exploiting the rules to our full advantage. I am not suggesting cheating. We need to review the rules and make sure we are doing every legal thing to make the air around us an ally. If you have tubing exposed, place some shaped foam on the downstream side to make air flow around the tube in a less turbulent way.

Are body panels mounted with hex-head bolts exposed to the air stream? Why not use a counter-sunk screw or a flush-mounted 1/4-turn fastener to secure a body panel? Cleaning up the surface of the body can and will make a big difference, not to mention it looks more professional to the other railbirds in the pits.

Look at the wheel cutouts. Are they too big? Cutouts that are too big are just big scoops sucking in air. Make sure the wheelwells are not larger than they need to be to conform to the rules, and then make sure they are not so small that they interfere with the tire through its suspension travel and the steering function. This is just good car preparation.

It is easier to make the wheel openings larger. If you have made the fender opening too big, it may be time to replace the fender and start over or replace the metal you have cut off. If your tires are sticking out past the edge of the fender, you will want to do everything possible to cover that tire. Remember that frontal area we are so concerned about? You have fenders, so use them to your advantage.

As stated, at the lower speeds seen at the short-track level, this is more common sense than rocket science. Preparation followed by execution will help you be the guy smiling after the race with the giant check.